This invention relates to the construction and manufacture of masks for soft X-rays. More particularly, it relates to the construction and manufacture of masks for exposure to soft X-rays which have a large exposure area and eliminate the possibility of any "dislocation of mask".
Conventionally, the photoengraving technique has been employed in the fabrication of semiconductor intergrated-circuit devices. However, the resolving power of the photoengraving technique is limited by the effects of interference and diffraction which are directly related to the wavelength of light used for that purpose, so that the minimum line width which can be reproduced stably is of the order of about 2 microns (.mu.). Accordingly, from a demand for miniaturization and large-scale integration of integrated-circuit devices, the techniques of exposure to scanning electron beams and to soft X-rays instead of light have been developed. However, the former technique requires much time because the object must be continuously scanned with an electrol beam. Therefore, it is useful for the manufacture of master masks but, from an economical point of view, not convenient for the purpose of processing semiconductor substrates directly. On the other hand, the latter technique, commonly called the "X-ray transfer technique", in which soft X-rays having a wavelength of several angstroms (A) is used instead of light, is inexpensive and economical as compared with the above-mentioned scanning electrol beams techinique. A mask for soft X-rays comprises a masking material and a film which supports the masking material and permits the penetration of soft X-rays. The thickness of the masking material is limited by the method used for patterning it. For example, the thickness is up to about 0.5.mu. when the minimum line width is 1.mu.. Correspondingly, the thickness of the film which permits the penetration of soft X-rays is at most about 10.mu., though it depends on the wavelength of soft X-rays and the material of the film.
For the film which permits the penetration of soft X-rays, films of silicon and of organic material have been used in the past. Films of organic material facilitate the manufacture of masks having a large exposure area, but have the fatal disadvantage f producing a "dislocation of mask" due to the difference between the thermal expansion coefficients of the film and the semiconductor substrate or silicon. Thus, the accuracy of masks will not be maintained unless precise temperature control is achieved both during the transfer process and during the mask manufacturing process.
In the case of silicon films, the above-mentioned problem of "dislocation of mask" does not arise because they are made of the same material as that of the substrate. They are readily used to make masks having a very small exposure area, but the manufacture of masks having a large exposure area has heretofore been considered to be difficult because they are brittle as compared with films of organic material. In order to overcome this difficulty, there has been proposed the method in which, as shown in FIG. 1, a mask pattern 2 is formed on one surface of a silicon substrate 1 having a thickness of about 10.mu. and a supporting member 3 is bonded to the other surface by means of an adhesive 4. Although masks having a relatively large exposure area can be obtained according to this method, the disadvantage of tending to produce a warpage or distortion of mask due to thermal expansion or shrinkage of the adhesive 4 makes this method of little practical value.